The present invention relates to a method for producing a methane-rich stream and a C2+ hydrocarbon-rich fraction from a dehydrated feed natural-gas stream, the method being of the type comprising the following steps:                cooling the feed natural-gas stream advantageously at a pressure greater than 40 bars in a first heat exchanger, and introducing the cooled feed natural-gas stream into a separator flask;        separating the cooled natural-gas stream in the separator flask and recovering an essentially gaseous light fraction and an essentially liquid heavy fraction;        forming a turbine input flow from the light fraction;        dynamically expanding the turbine input flow in a first expansion turbine and introducing the expanded flow into an intermediate portion of a splitter column;        expanding the heavy fraction and introducing the heavy fraction into the splitter column, the heavy fraction recovered in the separator flask being introduced into the splitter column without passing through the first heat exchanger;        recovering, at the bottom of the splitter column, a bottom C2+ hydrocarbon-rich stream intended to form the C2+ hydrocarbon-rich fraction;        sampling at the head of the splitter column a methane-rich head stream;        heating up the methane-rich head stream in a second heat exchanger and in the first heat exchanger and compressing this stream in at least one first compressor coupled with the first expansion turbine and in a second compressor for forming a methane-rich stream from the compressed methane-rich head stream;        sampling in the methane-rich head stream a first recirculation stream; and        passing the first recirculation stream into the first heat exchanger and into the second heat exchanger in order to cool it down, and then introducing at least one first portion of the first cooled recirculation stream into the upper portion of the splitter column.        
Such a method is intended to be applied for building new units for producing a methane-rich stream and a C2+ hydrocarbon fraction from a feed natural-gas, or for modifying existing units, notably in the case when the feed natural-gas has a high ethane, propane and butane content.
Such a method also applies to the case when it is difficult to apply cooling of the feed natural-gas by means of an outer cooling cycle with propane, or to the case when the installation of such a cycle would be too expensive or too dangerous, such as for example in floating plants, or in urban regions.
Such a method is particularly advantageous when the unit for fractionating the C2+ hydrocarbon cut which produces the propane intended to be used in the cooling cycles is too far away from the unit for recovering this C2+ hydrocarbon fraction.
The separation of the C2+ hydrocarbon fraction from a natural gas extracted from the subsoil gives the possibility of satisfying both economic imperatives and technical imperatives.
Indeed, the C2+ hydrocarbon fraction recovered from natural gas is advantageously used for producing ethane and liquids which form raw materials in petrochemistry. Further, it is possible to produce from a C2+ hydrocarbon cut, C5+ hydrocarbon cuts which are used in oil refineries. All these products may be economically valued and contribute to the profitability of the facility.
Technically, the requirements of natural gas marketed in a network include, in certain cases, a specification at the level of the calorific value which has to be relatively low.
Methods for reducing C2+ hydrocarbon cuts generally comprise a distillation step, after cooling the feed natural-gas in order to form a methane-rich head stream and a C2+ hydrocarbon-rich bottom stream.
In order to improve the selectivity of the method, sampling a portion of the methane-rich stream produced at the head of the column after compression and reintroducing it after cooling into the column head are known for forming a reflux of this column. Such a method is for example described in US 2008/0190136 or in U.S. Pat. No. 6,578,379.
Such methods give the possibility of obtaining ethane recovery of more than 95% and in the latter case, even more than 99%.
Such a method however does not give entire satisfaction when the feed natural-gas is very rich in heavy hydrocarbons, and notably in ethane, propane and butane, and when the inlet temperature of the feed natural-gas is relatively high.
In these cases, the amount of cooling to be provided is large, which requires the addition of an additional cooling cycle if maintaining good selectivity is desired. Such a cycle consumes energy. Further, in certain facilities, notably floating facilities, it is not possible to apply such cooling cycles.
An object of the invention is therefore to obtain a method for recovering C2+ hydrocarbons which is extremely efficient and highly selective, even when the content of these C2+ hydrocarbons in the feed natural-gas increases significantly.